Twelve states require the consent of every party to a phone call or conversation in order to make the recording lawful.
These "two-party consent" laws have been adopted in
California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Montana, Nevada, New Hampshire, Pennsylvania and Washington (Hawai'i is also in general a one-party state, but requires two-party consent if the recording device is installed in a private place). Recording Phone Calls and Conversations | Citizen Media Law Project
Remember that Linda Tripp was charged in Maryland for illegally recording her conversations with Monica Lewinsky.